Seasonal Dynamics in Mycorrhizal Colonization and Fine Root Features of the White Poplar (Populus alba L.) in Natural Temperate Riverside Forests with Two Contrasting Soils

Abstract

Fine roots are the most dynamic and physiologically active components of belowground tree organs. However, much remains unknown regarding the changes in fine root morphological characteristics during mycorrhizal colonization, especially in natural sites. The aim of this study was to analyze seasonal heterogeneity in fine roots and the mycorrhizal colonization of mature white poplar (Populus alba L.) trees under different soil conditions. Two floodplain forests were selected in Central Europe (Poland), which differed in soil moisture and structure. Fine roots were sampled during one growing season from the upper soil layer. Poplars were characterized by dual mycorrhizal colonization on one root system. It was, therefore, possible to investigate the contribution of two mycorrhizal types (arbuscular mycorrhiza—AM; and ectomycorrhiza—ECM) in response to different habitat conditions. The season was shown to be significant for all fine root features, as well as the degree of mycorrhizal colonization. Roots were better adapted to a drier habitat with a greater proportion of sand, mainly due to a reduction in the fine root diameter (FRD), while other root characteristics did not differ significantly. The degree of mycorrhizal colonization (RLC) and the proportion of arbuscular mycorrhizal structures (AM) were significantly and negatively correlated with the soil water content. A mutual competition between arbuscular mycorrhizas and ectomycorrhizas for poplar roots was also observed, particularly with respect to the season, site, and soil moisture. Changing environmental conditions (especially soil moisture) contribute not only to the morphological and functional changes of fine roots but also to changes in the proportion of arbuscular mycorrhiza and ectomycorrhiza. Understanding the mechanisms of adaptation of tree roots to changing environmental conditions is especially important in the context of climate change.